expired nodes: rebuild the subtree on reads

[gballet]

I decided not to rebuild the tree on reads, in order to make a potential resurrection attack more expensive.

If it turns out this is necessary, here is the diff:

@@ -272,8 +238,22 @@ func (t *Trie) get(origNode node, key []byte, pos int) (value []byte, newnode no
                        return nil, n, false, fmt.Errorf("failed to resolve expired node: %w", err)
                }
                newnode, err := archiveRecordsToNode(records)
-               value, newnode, _, err = t.get(newnode, key, pos)
-               return value, newnode, true, err
+               for _, record := range records {
+                       // make sure that the path up to the node matches
+                       if bytes.HasPrefix(key[pos:], record.Path) {
+                               resolved, err := decodeNodeUnsafe(nil, record.Value)
+                               if err != nil {
+                                       return nil, n, false, fmt.Errorf("failed to deserialize RLP node: %w", err)
+                               }
+                               if leaf, ok := resolved.(*shortNode); ok {
+                                       // make sure that the key to the leaf also matches
+                                       if bytes.Equal(key[pos+len(record.Path):], leaf.Key) {
+                                               return leaf.Val.(valueNode), newnode, true, nil
+                                       }
+                               }
+                       }
+               }
+               return value, newnode, false, err